1. Field of the Invention
As is well known, potassium is one of the three chemical elements that is essential to proper nutrition of living plants. The most common form of potassium applied to soil systems is the chloride salt, however, in some crop systems, such as tobacco, the chloride salt is to a degree toxic to the living plants; consequently, the choice potassium salt is the sulfate. In 1987, the estimated consumption of K.sub.2 SO.sub.4 and KCl salts used by the agricultural industry was about five million short tons and of that quantity, about 40 percent was in granular form and about 60 percent was finely divided particulate material classified as either standard, coarse, or suspension-grade potash materials.
The demand for granular potash fertilizers was fostered with the advent of "bulk blending." In this now large segment of the fertilizer industry, the granules of different composition are blended in proportions calculated to yield a mixture of the desired fertilizer nutrient composition. Bulk blending is extremely popular in the United States and is rapidly gaining worldwide popularity. However, to ensure the homogeneity within a given small volume of such dry-blended fertilizers during mixing, handling, and field application; it has been found essential that all the various ingredients be of closely-matched, particle-size distribution (Hoffmeister, George. "Quality Control in a Bulk-Blending Plant," Proc. TVA Fertilizer Bulk-Blending Conference, Louisville, Ky., Aug. 1-2, 1973). Ignoring this requirement and preparing blends from ingredients of unmatched particle-size distribution inevitably results in segregation of the various components during the mixing or preparation step and subsequently during handling, and distribution such as for example, field application. Thus, when blends are prepared from fertilizer materials of unmatched particle-size distribution, homogeneity is lost with resultant undesirable agronomic effect.
In view of this important requirement that individual ingredients of bulk blends be matched in particle-size distribution, finely divided particulate potash materials classified as standard, coarse, or solution-grade materials cannot satisfactorily be incorporated therein. In order to meet the demand for granular potash materials, the potash industry utilizes the so-called Mannheim process or a modification thereof for production of K2SO4 and a high-cost preheat-compaction-crush and sizing procedure for production of granular KCl.
2. Description of the Prior Art
In production of K.sub.2 SO.sub.4 by the Mannheim process, potassium chloride (KCl) is reacted with sulfuric acid to produce potassium bisulfate (KHSO.sub.4), subsequently the bisulfate together with additional KCl is heated in a Mannheim furnace to convert the bisulfate and the additional KCl to sulfate (K.sub.2 SO.sub.4). The K.sub.2 SO.sub.4 clinkers formed in the furnace are then removed, crushed, and sized by screening; the various resulting particle size fractions are marketed as either granular, standard, or suspension-grade products.
In the production of granular KCl, fine-size KCl is first preheated and then fed to a pair of high-pressure rolls that turn toward each other to compress the material into sheets or into briquettes. The resulting compacted material is subsequently crushed and screened, and the undersize material is recycled for additional heating, compaction, and processing. The processes currently used for production of granular K.sub.2 SO.sub.4 or KCl are not completely satisfactory for meeting all of the fertilizer industries' requirements in that capital costs are high and energy requirements and maintenance costs are excessive. Thusly, a substantial premium can be, and usually is, demanded for granular type potassium fertilizers. In view of these considerations, it should now be obvious to those skilled in the art just how important the present invention is to the large class of bulk-blended fertilizers which need and require granular type feedstock.